When large diameter, relatively stiff pipes are lowered into an undersea trench, the usual elastic curvature is such that an unacceptably long span develops, and the corresponding open section of the trench may be too long to be practical unless stable slopes have been cut. For example, if a 12-foot deep trench were in a sandy soil, the slope of the trench sidewalls might be on the order of one in three, and the amount of soil to be excavated could be five times that of a vertical sidewall ditch. Accordingly, it is not only desirable to reduce the volume of soil to be excavated, but further to prevent infilling of the soil until the pipe has reached the ditch, both to reduce power requirements and save time in construction. One solution to this problem is disclosed in application Ser. No. 562,363 filed Dec. 16, 1983, wherein an underwater trencher with pipelaying guide is provided for the simultaneous trenching and installation of submarine pipelines by a technique which greatly reduces the volume of soil to be excavated by preventing infilling until the pipe has reached the bottom of the trench, which trencher is relatively convenient and economical to use and saving of construction time, and which greatly reduces power requirements. First, a trench is formed while simultaneously an elongated shield is positioned in the trench to prevent the sidewalls of the trench from collapsing. Then, a pipeline is deflected into the trench so that the pipeline enters the trench at one end of the shield and approaches the bottom of the trench before exiting at the other end of the shield. Preferably, the pipeline is subjected to bending strain, for example, in a range of about 0.002 to 0.003 or greater, which greatly reduces the length of shield required, which nonetheless is a large piece of equipment, typically over 100 feet in length. If the shield is pulled (e.g. with cables) to overcome soil resistance (friction and cutting head pressure), the draw force required is in the range of hundreds of kips (1 kip=1,000 lbs). Accordingly, it is desirable to provide means for reducing or obviating these extreme requirements of pulling force necessary for advancing the shield.